U.S. Patent Documents
Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates to the fabrication of articles from nanoparticulate materials. More particularly, the present invention relates to a method to overcome the problems of the prior art in controlling aggregation, contamination and pyrophoricity during handling of nanoparticulate materials and the fabrication of articles therefrom.
2. Description of Prior Art
In a broad sense, nanomaterials, nanocrystalline or nanostructured materials or simply nanostructures refer to dense materials with grain sizes in the nanometer (one billionth of a meter) range. The designation xe2x80x98nanoparticulatesxe2x80x99 is generally applied to any particulate matter with an average dimension below one micrometer. In the literature, the terms nanoparticulates, submicrometer powders, nanopowders, nanoscale powders or nanocrystalline powders are often used interchangeably. The terms nanosuspension or nanodispersion usually refer to suspensions of discrete nanoparticulates, in either a liquid or in a solid matrix.
Nanoscale powders are not new. The use of lampblack, a carbon nanoscale powder with particle sizes in the 10-100 nm, to make Chinese ink, predates the Christian era by thousands of years. Nanoscale metal oxides have been used in the paint industry for centuries, whereas nanoscaled silica powders are used as filler additives to tailor the rheological properties of a variety or organic suspensions. In the hardmetal industry ultrafine carbide and nitride powders are used to make cutting tools with increased strength and extended economic life over those produced from conventional powders. More recently, the use of oxide nanopowders in optics, electronic, and in cosmetics for UV protection is well established.
It is well known that a decrease in particle size results in enhanced sintering kinetics of particulate materials. When particle size reaches the nanometer range, full densification is often possible at substantially lower temperatures than those needed for sintering coarse-grained particulates. This is because nanoparticles imply shorter diffusion lengths while promoting boundary diffusion mechanisms. In addition to savings in energy, lower sintering temperatures also result in reduced contamination, stresses and cracking during cooling.
The enhanced sintering kinetics of nanoparticulate materials are already exploited in the microelectronic packaging industry, where metal alloy nanopowders are incorporated in cold-weldable welding pastes to achieve ductile and electrically conductive metal to metal bonds.
In the refractory metal industry, a decrease of several hundreds of degrees in the sintering temperature is achievable when standard 2 xcexcm tantalum powder used to produce tantalum capacitors is replaced with a 50 nm nanopowder.
Aside from geometric considerations, the prefix xe2x80x98nanoxe2x80x99 also implies dramatically improved material properties as inferred from the well-known Hall-Petch relationship according to which a material""s strength increases proportionally to the inverse square root of its grain diameter.
Hence, interest has been growing in nanoparticulate materials stemming from the fact that novel phenomena are being discovered at the nanoscale level, and there is immense potential for improving structural and functional properties of components and devices by xe2x80x98nanostructuringxe2x80x99 as nanostructures can generate superplastic or ultra-high strength, tough materials. Extrapolations based on reducing grain size have produced forecasts of 2-7 times higher hardness and 2-3 times the tensile strength of parts produced from conventional powders.
For example the yield strength of an 80 nm iron nanopowder sintered to 99.2% of theoretical density is about 2.4 GPa, roughly five times that of conventional iron with a particle size in the 25 xcexcm range.
The improved material properties of nanostructures have already found applications in many different fields of industry and technology. For example nanograined powders are already used in hydrogen storage technology. Another fast growing field of application is that of nanopowder-polymer composites for microelectronic applications. Using metallic nanopowders dispersed in polymers allows the fabrication of electrically conductive adhesives, radio frequency shielding polymers, and magnetic polymeric layers. Another area of strong interest is the fabrication of lightweight electrical wires using conductive nanopowders in a polymer matrix. These extrinsically conductive polymer wires with nanoparticulate fillers exhibit improved electrical percolation. Since the volume of filler material needed to provide conductivity can be reduced by over 50%, the intrinsic flexibility, strength and toughness of the polymer matrix material is retained.
The use of nanopowders as reinforcing phase in nanocomposites is a fast developing technology where the vastly increased interfacial area between the nanoparticles and the matrix material leads to improvements in the amount of energy absorbed during mechanical stress. This is especially useful in applications such as ballistic armor protection, where improved energy absorption under high strain rate conditions leads to increased ballistic impact resistance. Furthermore the reduction in the amount of filler phase necessary to reinforce the polymer matrix reduces overall component weight.
The fine size of nanoparticulates also allows for the design of strengthened optically transparent components such as aircraft canopies. In this case the nanoparticle reinforcing agent is so fine that interference with the wavelengths of the visible light spectrum is minimized or eliminated.
Another exciting field of application is that of lithium ion batteries where nano-vanadium pentoxide has been shown to possess electrochemical properties that are different from those of commercial coarse-grained V2O5 powders, and these properties can be attributed to the structure of the nanoparticles. The discharge-charge voltage curve of nano-V2O5 is continuous whereas, in contrast, coarse-grained V2O5 has a stepwise curve which is unsuitable for lithium ion batteries. For the same number of discharge charge cycles, the capacity of nano-V2O5 is 60% higher than that of commercial powder. Furthermore irreversible losses are also much smaller when using nano-V2O5.
In the area of materials joining, copper, gold, nickel, tin and solder powders are routinely formed into pastes and used for electronics interconnects. The pastes are printed on ceramics such as aluminum oxide, and more recently aluminum nitride, to produce highly dense, so called thick film circuits. The requirement to shrink circuits and increase functionality has resulted in a continuing search for new and improved processes. One of the latest developments is in copper based pastes that can be applied to ceramic substrates at temperatures substantially below those currently used to manufacture thick film circuits. Lower temperatures are desirable because many electronic components are degraded by excessive temperatures. The lower sintering temperatures also allow environment-unfriendly lead based solder pastes to be phased out. Nanoparticulate-based joining formulations offer the potential to tailor the metallurgy and to lower the brazing temperature.
Nanoaluminum powders are also advantageously used in solid propellant formulations, doubling the burning rate as compared with that of compositions based on micrometer size aluminum. High burning rates increase thrust and speed of response. Adding nanoaluminum to hydrogen and kerosene burning with liquid oxygen increases the amount of energy released, resulting in smaller fuel tanks and shorter, lighter weight rockets.
There are many methods to produce nanopowders, most of them recent developments. Conventional powder fabrication techniques such as gas or water atomization fail in the submicrometer range, as they usually have a lower particle size limit of 1-5 xcexcm. The past decade has seen significant effort in developing technology for synthesizing and processing materials at the nanometer scale.
One of several newer approaches is the inert gas condensation (IGC) method, which consists of evaporating and condensing the respective material in a vacuum chamber with a low partial pressure of inert gas (e.g. 10 mbar helium). With IGC, high quality powders with low chemical impurity levels and low contents of oxides or nitrides from the production process can be produced.
In a variation of this process metal particles are embedded in an organic matrix by evaporating both the metal and an organic liquid in the same vacuum vessel and subsequent co-condensation of the metal and the organic vapor under inert gas or vacuum. This process allegedly enables production of metal particles without agglomeration or oxide layers in an organic suspension. Particle size distribution is narrow, with a mean particle size in the range 2-50 nm.
Another process, the flame or plasma reduction method, uses the decomposition and reduction of metal salts in a gas flame or plasma. Sodium reduction of halides has been standard industrial practice for decades for synthesizing materials like titanium and zirconium. In one variation, the sodium/halide flame and encapsulation (SFE) technique uses sodium reduction of metal halides to produce the metal or, if the reaction occurs in the presence of a non-metal, a ceramic. The byproduct of the chemistry is salt, typically sodium chloride, which is used to encapsulate the particles within the flame.
Other processes include the chemical vapor reaction (CVR) method, which uses the reaction of metal chlorides and hydrogen in a hot wall reactor and the combustion flame chemical vapor condensation (CF-CVC) process. The latter purportedly has the ability to minimize the extent of particle aggregation. Still other techniques used are the pulsed plasma jet process and the electro-explosion of wire (EEW) process where an electrical pulse is applied to a wire.
Finally, mechanical milling or attriting, probably the oldest and best known of comminuting techniques, can produce large quantities of nanocrystalline materials with grain sizes below 100 nm from commercial coarse-grained metallic or ceramic powders as the starting materials. Mechanical milling is attractive because it has the advantage of being a simple and inexpensive process usually performed at room temperature and which can be readily scaled up for mass production.
Virtually all the problems in the production and processing of nanopowders stem from their high reactivity and worsen with decreasing particle size.
As the main quality issue, the pick-up of oxides or nitrides during processing or storage can be a serious problem. In most cases the incorporated oxygen has deleterious effects on the mechanical properties of the end product. To reduce contamination, nanopowder production and consolidation operations are mostly performed under inert gas atmosphere.
The purity requirement of nanoparticulate materials is application dependent as well. For advanced materials a specification of total metallic impurities less than 100 ppm is common.
Nanoparticulate materials have a strong tendency to form aggregates or agglomeratesxe2x80x94most authors use the terms indiscriminatelyxe2x80x94which have a deleterious effect on powder processibility and end product quality. This is particularly relevant in applications such as rechargeable batteries, where a high packing density of the nanoparticulates is essential.
The high reactivity of nanoparticulate materials also leads to undesirable grain growth. The driving force for grain growth increases as grain sizes decreases, such that the advantage nanophase materials have in sinterability can be lost due to concomitant grain growth that destroys the desirable nanoscale grain size, defeating any efforts to form nanostructures.
Therefore, to maintain nanoscale grain size, consolidation temperatures often have to be limited to below 600xc2x0 C., and typically to the 400-500xc2x0 C. range.
Early work on the consolidation of metallic nanophase powders employed exotic techniques, such as shockwave compacting, to overcome the difficulties in maintaining nanophase grain size. Subsequent attempts using various techniques such as hot pressing, hot extrusion, sinterforging, hipping, etc. produced consolidated products that are either porous (around 90% of theoretical density), or fully dense but at the expense of the nanophase microstructure.
Virtually all nanoparticulate materials are pyrophoric and must be handled and shipped as hazardous materials. Materials that are relatively stable as micrometer-sized powders can become dangerously explosive when in nanopowder form. Even when the powders do not react violently they may still pick up excessive oxygen, which usually makes their properties less desirable.
The cost of nanoscale powders is presently in the hundreds of dollars per kilogram range. While substantial reductions in cost are anticipated as nanotechnology develops, nanopowders will always be more expensive to produce than micrometer sized powders and entail much higher shipping and handling costs.
The production rate capabilities of any specific nanopowder production process and the stringent material specifications such as the primary particle size, size distribution, purity and extent of aggregation, are of paramount importance in determining the cost of the nanopowders. In the end powder cost which will ultimately be the decisive factor in determining whether nanomaterial-based commercial applications can be economically competitive.
In short, although there are numerous methods for producing nanopowders, most are not practical from a commercial perspective.
The powders must not only be of the desired size and morphology but also stable in air so that they can be handled and processed without excessive oxygen contamination or safety risks. Methods to avoid contamination and aggregation should be consistent with conventional industrial practices of the powder processing industry. For example, it would be impractical to require that nanopowders be continuously handled and processed in ultrahigh vacuum. Also the need to eliminate impurities, contaminants or aggregates can dramatically increase powder cost. For instance, salt encapsulation, used to protect nanoparticles produced by the sodium/halide flame and encapsulation (SFE) technique, imperatively requires that the salt be removed during subsequent processing using water, ammonia, an appropriate solvent or even vacuum sublimation at 700xc2x0 C.
In accordance with the present invention the problems of the prior art are substantially overcome by providing an economic process to generate optimally-sized nanopowders substantially free of aggregates and contamination, while reducing or eliminating the risks associated with their inherent pyrophoricity, and integrating said process in subsequent consolidation techniques for mass-production of dimensionally accurate nanostructures for commercial use.
It is a primary object of this invention to provide an integrated method for the fabrication of nanoparticulate materials free from aggregates and contamination.
It is another object of this invention to provide an integrated method for the fabrication of fine-grained nanostructures, substantially free from contamination and impurities. The fine-grained nanostructures are produced by first shaping green parts from an optimized dispersion of said aggregate and contamination-free nanoparticulates in a thermoplastic binder. All process steps, from dispersing the nanoparticulates into the organic binder through removal of the organic binder from the green parts to sintering of the resulting binder-free performs are performed contiguously and under protective atmosphere. The fine grain microstructures are achieved by sintering at the lowest possible temperature. At no time during this manufacturing process are the nanoparticulates or green parts exposed to temperatures which would promote excessive grain growth.
It is a particular object of this invention to provide a manufacturing process for nanostructures that are substantially sodium-free. This is a critical requirement for many applications in the microelectronic and semiconductor industry.
It is yet another object of this invention to provide a manufacturing process for nanostructures with improved dimensional accuracy.
It is still another object of this invention to provide an economic manufacturing process for nanostructures which lends itself easily to automation and mass-production and makes use of well-known prior art techniques such as casting, machining, molding, sintering, etc. It is therefore also an object of this invention is to provide a manufacturing method for nanostructures which is easily accessible to the nanufacturing industry instead of being restricted to specialized industries, laboratories and academic establishments. Hence, through this invention the world of nanotechnology will be opened to a wider number of practical users.
Still another object of this invention is to provide a method to optimize the selection and use of surfactants for nanoparticulate materials.
Also an object of this invention is to provide a method for controlling aggregation of nanoparticulate materials.
It is a further object of this invention to provide a method to optimize the dispersion of nanoparticulate materials in a thermoplastic organic matrix or binder.
An additional object of this invention is to provide a method to control the pyrophoricity inherent in nanoparticulate materials.
Still another object of this invention is to provide a method to fabricate nanostructures without the cost and material limitations inherent to the prior art. Through the use this invention nanostructures can be produced from a virtually unlimited number of material compositions.